Complexation of iron (III) to humic substances of a humic podzol at pH 2.5 ‐ 6.4 ‐ quantification of the organically complexed iron by pyrophosphate extraction

Bonding of Fe(III)‐ions to humic substances of a humic podzol are reported with regard to pH (range 2.5 ‐ 6.4). A separation technique is used involving a pyrophosphate extraction, whereby dissolved, precipitated, and organically bound iron can be separated. The necessity of iron speciation results from the fact that the amount of Fe bound to organic matter is overestimated if conventional methods are used. The estimation of formation constants for iron actually bound to organic matter leads to a corrected maximum value of log K'f(org) = 1.64, whereas for the total eliminated Fe this maximum value is 2 units higher (log K'f= 3.70).